AND PHYSIOLOGY. 



381 



muscles attached to the integuments, whenever it is desired 

 they bring the brows in front of and above the eye in such a 

 manner as to afford it a very considerable protection from ex- 

 cessive light. 



699. Short and Long sightedness. — The phenomena of 

 short and long-sightedness are worthy of a passing notice. 

 Short-sightedness results where the lens of the eye has too 

 much convexity, which causes the image to be formed in front 

 of the retina. In such persons the eye generally has greater 

 prominence than in others. But persons thus affected will 

 probably in later years have better eye-sight than if short- 

 sightedness had not existed. A too much flattened state of 

 the lens occurs in those who are long-sighted. This is gener- 

 ally the case with those somewhat advanced in life. In this 

 case the distinct image will be formed behind the retina, and 

 it requires a convex lens, in the form of spectacles, to correct 

 the error. 



700. Other Interesting Phenomena of Vision,— A few 

 other phenomena connected with the function of vision de- 

 serve attention. One is the fact, that although Ave have two 

 eyes and two distinct images are transmitted to the brain, 

 yet but one object is seen. And if we look at an object with 

 only one eye, we see the image nearly as distinct as with both 

 eyes. Another is, that although the image on the retina is 

 inverted, yet to the mind it appears in its true position. For, 

 since the rays of light pass through the lens in nearly straight 

 lines, the ray coming, for instance, from the top of a tree, will 

 fall upon the lowest part of the retina, while the ray coming 

 from the bottom will strike the upper part, and hence the 

 image will be inverted, although to the u mind's eye" it will 

 be erect. Another fact is equally wonderful. We are able 

 to see with distinctness an object only a few inches from our 



699. What is the cause of short-sightedness? What is the reason of long-sightedness ? 

 What kind of a lens is required for long-sighted people, and why ? TOO. What curious 

 facts connected with the physiology of vision ? 



